The present invention relates to a method of manufacturing a flexible polyurethane foam product and, more particularly, to a method of manufacturing a patterned cushioning product of flexible polyurethane foam comprising a sheet of flexible polyurethane foam and surface covering sheets fusion-bonded respectively to the opposed surfaces of the polyurethane foam sheet, which product is generally known as a weldable pad material suitable for use as a flooring or carpetting material, wall lining material or any other surface finishing or lying material.
As is well known to those skilled in the art, a flexible polyurethane foam is now commercially available in two types depending upon the pattern of distribution of the gas phase in voids or pockets which are generally called cells. One type of flexible polyurethane foam is of open-celled structure wherein the cells are interconnected in such a manner that gas may pass from one to another, whereas the other type is of closed-celled structure wherein the cells are discrete and the gas phase of each is independent of that of the other cells. Of these two types, the flexible polyurethane foam of open-celled structure is applicable in the method of the present invention.
There is known a method of manufacturing a patterned cushioning product of polyurethane foam of open-celled structure which comprises the steps of permeating a powder of thermally fusible bonding agent locally into the open-celled structure of a flexible polyurethane foam sheet, applying a heat of a temperature approximating to the melting point of the thermally fusible bonding agent to fix the particles of the bonding agent in the open-celled structure of the flexible polyurethane foam sheet, and fusion-bonding surface covering sheets to the opposed surfaces of the flexible polyurethane foam sheet while a portion of the assembly where bonding is desired to be effected is applied with a pressure.
The above described method is substantially disclosed in the Japanese Patent Laid-open Publication published on June 21, 1975 under No. 50-76181 and, according to it, three processes have been discussed as to a method of permeating the particles of the powdery bonding agent into the open-celled structure, that is, the interconnected cells, of the flexible polyurethane foam sheet, namely, to vibrate the flexible polyurethane foam sheet to allow the bonding particles previously spread over one surface of the polyurethane foam sheet to permeate into the open-celled structure; to employ a high pressure necessary to drive the bonding particles into the open-celled structure of the polyurethane foam sheet in a direction in which the high pressure is applied; and to employ a suction force necessary to draw the bonding particles into the open-celled structure from one of the opposed surfaces of the polyurethane foam sheet towards the other in a direction in which air is drawn.
Both of the last two processes require the employment of a relatively large amount of the powdery bonding agent and correspondingly a recovery equipment for recovering some of the powdery bonding agent which has completely passed through the thickness of the polyurethane foam sheet. Moreover, in the practice of any of these permeating processes, some of the powdery bonding agent permeated into the open-celled structure of the polyurethane foam sheet tend to aggregate in localized areas adjacent the opposed side edges of the polyurethane foam sheet and, therefore, uniform distribution of the bonding particles in the open-celled structure of the polyurethane foam sheet can hardly be achieved with no difficulty.
In view of the above, when the polyurethane foam sheet permeated with the bonding particles is, during the subsequent manufacturing procedure, subjected to a high frequency bonding process wherein the surface covering sheets are bonded thereto by the application of high frequency waves, the resultant flexible polyurethane foam product shows that side portions thereof corresponding to the localized areas of the polyurethane foam sheet where some of the bonding particles have been aggregated are hardened with physical properties intrinsic of the polyurethane foam, particularly, the impact resilience, being reduced accordingly.